Rollup merge of #125699 - nnethercote:streamline-rustfmt, r=GuillaumeGomez

Streamline `x fmt` and improve its output

- Removes the ability to pass paths to `x fmt`, because it's complicated and not useful, and adds `--all`.
- Improves `x fmt` output.
- Improves `x fmt`'s internal code.

r? ``@GuillaumeGomez``

Author: fmease